Various methods are known for improving images that are called blurred images and lack sharpness. In many cases, the methods consist of sharpening edges in an image by enhancing high-frequency components of a video signal. In this case, if the enhancement is insufficient, the sharpness of the image becomes insufficient, giving a blurred impression. However, if the enhancement is excessive, overshoot occurs in a pixel signal at an edge, and noticeable edging occurs, giving an unnatural impression.
Therefore, in the related art, according to the state of an original image, how much the outline enhancement is appropriate is made by using some evaluation functions or human judgment.
In addition, in order to achieve natural outline enhancement, a frequency characteristic of a filter is selected according to the contents of the image by using a high-pass filter, and in particular, the quality of the image is optimized by adjusting a gain of the high-frequency component. For example, in PTL 1, it is proposed to obtain a preceding signal (S1), a target signal (S2), and a delayed signal (S3) by processing an input signal by using two delay means provided in series with each other and set a value with the smallest absolute value among the secondary differential signals which are given as a difference between the target signal (S2) and the preceding signal (S1), a difference between the delay signal (S3) and the target signal (S2), and a difference between the target signal (S2) and an average value of the preceding signal (S1) and the delayed signal (S3), to which a sign of the secondary differential signal is added, as an outline enhancement signal. In PTL 2, a technique of calculating a maximum value and a minimum value of densities of pixels of a target small area and suppressing an overshoot amount due to an outline enhancement processing according to the maximum and minimum values.
However, generally, an image includes areas having various different characteristics, and in a case where specific outline enhancement is performed, the image quality improvement result varies depending on the areas. That is, generally, the image includes an area which is called a texture area where the density gradually changes regularly due to the properties of the surface of an object, edges are distributed complicatedly and an edge area including a clear edge such as a line or outline. Any area is an important factor in improving the quality of the image. Therefore, in order to enhance a video image, it is desirable that such an area is sharpened, and discomfort is not caused.
In addition, since it is highly necessary to enhance a video image in real time, it is desirable that suitable outline enhancement is possible without requiring a large amount of calculation.